1. Field of the Invention
The present invention relates to lithium batteries, and in particular relates to a lithium battery with a silicon negative electrode and an electrolytic solution thereof.
2. Description of the Related Art
A lot of research regarding batteries as a driving energy source has been conducted to minimize battery weight for, and meet sophisticated technological requirements of, portable electronic devices such as video cameras, cellular phones and laptop computers. Particularly, the rechargeable lithium batteries have more energy density per unit weight then conventional lead storage batteries, nickel-cadmium batteries, nickel-hydro batteries and nickel-zinc batteries. In addition, they can be quick recharged.
Electrolytes of the lithium battery are categorized as liquid or solid electrolytes, according to the electrolytic type. However, the liquid type electrolyte often raises many safety problem issues including the potential danger for fire due to leakage, and outflow and destruction of batteries from evaporation. Hence, many researchers have suggested using solid electrolytes instead.
Many studies have particularly focused on solid polymer electrolytes, because solid polymer electrolytes are unlikely to leak electrolytic solution, and they are easy to process. Solid polymer electrolytes are further categorized into full solid types and gel types, wherein the full solid types do not contain an organic electrolytic solution, while the gel types do.
Generally, conventional aqueous electrolytic solutions are not suitable for lithium batteries mainly because they may react violently with lithium, which is used as an anode. Thus, an organic electrolytic solution in which a lithium salt is dissolved is used instead. The organic solvent may have high ionic conductivity, a high dielectric constant and low viscosity. But it is very difficult to obtain a single organic solvent having all three of these characteristics. As a result, a mixed solvent composed of an organic solvent having a high dielectric constant and an organic solvent having a low dielectric constant, or a mixed solvent composed of an organic solvent having a high dielectric constant and an organic solvent having low viscosity, is used as is an organic solvent for lithium batteries.
A lithium battery positive electrode is typically composed of an active material including transition metal compounds such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1-xO2, Ni1-x-yCoxMnyO2, or oxides containing the transition metal compounds and lithium. A lithium battery negative electrode is typically composed of an active material including lithium metal, a lithium metal alloy or a carbonaceous material, and a graphite material. However, the electric capacities of the described negative electrode materials are lower than that of the silicon negative electrode material, wherein electric capacity may reach 4,684 mAh/cc. The silicon source can be inexpensive quartz sand without doubt of environment pollution, and be an ultimate negative electrode material. Although the silicon material is advantageous compared to conventional negative electrode materials, several fabrication technique difficulties exists. First, the lithium ions of the electrolytic solution and the silicon negative electrode will alloy and de-alloy during charging and discharging, respectively. The re-crystallized silicon forms particles which are further ablated from the negative electrodes. Second, the volume of the negative electrode dramatically changes, up to over 200%, during alloying and de-alloying, such that the electrode structure peels due to relaxing of the adhesive agent of the electrode. Third, the silicon electrode alloys at a first charge/discharge and the re-crystallized silicon surface and the electrolyte reacts to form an SEI film, thereby resulting in irreversible electric capacity loss that is too high.
The principal methods for improving the charge/discharge efficiency of the silicon negative electrode are to change the electrode composition. In JP Pub. No. 2004-185984, the amorphous silicon content ratio in silicon negative electrode is enhanced. In U.S. Pat. No. 6,541,156, the silicon powder for manufacturing the silicon negative electrode is covered by carbon film.
Accordingly, a method for further improving the charge/discharge efficiency of the silicon negative electrode of the lithium battery is called for.